Field of the Invention
The present invention is related to a diaphragm pump and in particular to a hydraulically driven diaphragm pump with an overfill limit assembly utilizing two springs having different spring constants.
Description of the Prior Art
Diaphragm pumps are pumps in which the pump fluid is displaced by a diaphragm. In hydraulically driven pumps, the diaphragm is deflected by hydraulic fluid pressure forced against the diaphragm. Such pumps have proven to provide a superior combination of value, efficiency and reliability. However, such pumps require safeguards to prevent a hydraulic oil overfill condition. For synchronous high pressure pumps, such conditions may lead to the piston striking the manifold and cause pressure spikes against the diaphragm that could cause the diaphragm to fail.
To prevent such failures, systems have been developed to limit overfill. U.S. Pat. No. 6,899,530 to Lehrke and Hembree, and assigned to Wanner Engineering, Inc., of Minneapolis, Minn., teaches an improved valve system to limit overfill. The system uses a stiffer spring than conventional pumps and also has a vent groove in the cylinder that allows for priming the hydraulic chamber. However, such systems may leak small amounts of oil in the pressure stroke at very high pressures. Even such small leaks may not be acceptable for certain applications, thereby limiting the utility of such a system to low pressure pumps.
A further system also developed by Lehrke and Hembree and assigned to Wanner Engineering, Inc., is disclosed in U.S. Pat. No. 7,090,474. This patent discloses a system that eliminates the vent groove and uses a soft spring that applies force to the diaphragm even when empty. This configuration allows the pump to prime without a vent groove. However, to prevent overfilling, a travel limiter is utilized on the valve spool that causes an increase in pressure when the hydraulic chamber is overfilled. Therefore, under some conditions, the pressure may rise sharply when the diaphragm is overfilled and may lead to stress on the diaphragm in such conditions.
It can therefore be appreciated that a diaphragm pump with an overfill limiter is needed that avoids the problems of the prior art. Such a system should achieve a low pressure drop across the diaphragm that allows oil priming without requiring a vent groove in the cylinder and should also prevent excessive overfill, but also avoids excessive pressure levels as may occur with a rigid travel limiter. Moreover, such a pump and system should be inexpensive, easy to manufacture and service, and should minimize stresses to the diaphragm to maintain high reliability. The present invention addresses these as well as other problems associated with diaphragm pumps.